CVE-2026-24684: FreeRDP Use-After-Free Vulnerability

CVE-2026-24684 Overview

CVE-2026-24684 is a Use After Free vulnerability in FreeRDP, a free implementation of the Remote Desktop Protocol (RDP). Prior to version 3.22.0, the RDPSND async playback thread can process queued Protocol Data Units (PDUs) after the channel is closed and internal state is freed, leading to a use after free condition in the rdpsnd_treat_wave function.

This vulnerability affects the sound redirection channel (RDPSND) which handles audio playback during RDP sessions. The flaw occurs due to improper thread synchronization during channel cleanup, where the asynchronous playback thread continues to access memory that has already been deallocated.

Critical Impact

Remote attackers can exploit this use after free vulnerability over the network without authentication, potentially causing denial of service through application crashes. The asynchronous nature of the vulnerability makes it particularly dangerous in production RDP environments.

Affected Products

• FreeRDP versions prior to 3.22.0
• Applications and systems using vulnerable FreeRDP libraries for RDP connectivity
• Linux, Windows, and macOS systems running unpatched FreeRDP clients

Discovery Timeline

• 2026-02-09 - CVE-2026-24684 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-24684

Vulnerability Analysis

The vulnerability exists in the RDPSND (Remote Desktop Protocol Sound) channel implementation within FreeRDP. This channel is responsible for redirecting audio from the remote desktop server to the local client. The use after free condition (CWE-416) occurs because the asynchronous playback thread continues processing queued PDUs even after the channel cleanup routine has freed the associated internal state structures.

The core issue stems from a race condition between the channel cleanup process and the async playback thread. When an RDP session terminates or the sound channel is closed, the cleanup routine frees memory structures while the playback thread may still be actively processing audio data from its message queue. This results in the thread accessing freed memory, leading to undefined behavior.

Root Cause

The root cause is insufficient thread lifecycle management in the RDPSND channel implementation. The cleanup_internals function was deallocating memory structures without first ensuring the async playback thread had been properly terminated and its message queue drained. This violates proper thread synchronization principles and creates a window where the thread can access invalid memory.

Attack Vector

This vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker controlling a malicious RDP server can manipulate the timing of RDPSND channel operations to trigger the use after free condition when a victim connects. The attack involves:

1. Victim connects to attacker-controlled RDP server
2. Attacker initiates sound channel communication
3. Attacker manipulates channel state to trigger cleanup while PDUs are still queued
4. Async playback thread processes queued PDUs against freed memory

The security patch introduces the rdpsnd_terminate_thread function to properly manage thread lifecycle:

static void rdpsnd_terminate_thread(rdpsndPlugin* rdpsnd)
{
	WINPR_ASSERT(rdpsnd);
	if (rdpsnd->queue)
		MessageQueue_PostQuit(rdpsnd->queue, 0);

	if (rdpsnd->thread)
	{
		(void)WaitForSingleObject(rdpsnd->thread, INFINITE);
		(void)CloseHandle(rdpsnd->thread);
	}

	MessageQueue_Free(rdpsnd->queue);
	rdpsnd->thread = NULL;
	rdpsnd->queue = NULL;
}

static void cleanup_internals(rdpsndPlugin* rdpsnd)
{
	if (!rdpsnd)
		return;

	rdpsnd_terminate_thread(rdpsnd);
	if (rdpsnd->pool)
		StreamPool_Return(rdpsnd->pool, rdpsnd->data_in);

Source: GitHub Commit Changes

The fix ensures the playback thread is terminated by posting a quit message to its queue, waiting for thread completion, and closing the handle before any memory deallocation occurs.

Detection Methods for CVE-2026-24684

Indicators of Compromise

• Unexpected FreeRDP client crashes during or after RDP session termination
• Memory access violations or segmentation faults in rdpsnd_treat_wave function
• Core dumps indicating use after free in RDPSND channel code paths
• Abnormal RDP sound channel behavior with rapid channel open/close sequences

Detection Strategies

• Monitor for FreeRDP process crashes with stack traces referencing rdpsnd_main.c or rdpsnd_treat_wave
• Implement application-level crash monitoring to detect repeated RDPSND-related failures
• Deploy memory safety tools (AddressSanitizer, Valgrind) in development environments to catch use after free conditions
• Analyze network traffic for unusual RDPSND channel manipulation patterns

Monitoring Recommendations

• Enable verbose logging for FreeRDP connections to capture channel lifecycle events
• Configure crash reporting to aggregate FreeRDP client failures for pattern analysis
• Monitor system logs for memory corruption indicators associated with RDP client processes
• Implement endpoint detection for abnormal RDP client behavior

How to Mitigate CVE-2026-24684

Immediate Actions Required

• Upgrade FreeRDP to version 3.22.0 or later immediately
• Review all systems and applications using FreeRDP libraries and schedule updates
• Consider disabling sound redirection temporarily if patches cannot be applied immediately
• Restrict RDP connections to trusted servers only until patching is complete

Patch Information

The vulnerability is fixed in FreeRDP version 3.22.0. The fix introduces proper thread termination handling that ensures the async playback thread is gracefully stopped before any memory deallocation occurs. Two commits address this issue:

• Primary Fix - Introduces rdpsnd_terminate_thread function
• Additional Fix - Thread cleanup refinements

For full details, see the GitHub Security Advisory GHSA-vcgv-xgjp-h83q.

Workarounds

• Disable sound redirection in FreeRDP connections using /sound:off or equivalent configuration
• Use network-level controls to restrict RDP connections to known, trusted servers
• Implement application whitelisting to prevent connections to potentially malicious RDP endpoints
• Consider using alternative RDP clients until patching is possible

# Disable sound redirection in FreeRDP
xfreerdp /v:server.example.com /u:username /sound:off

# Or via configuration file
# Set AudioRedirection = 0 in FreeRDP configuration